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Full text of "Handbook Of Chemical Engineering - I"

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WS(Q - Ao) = Ww(h2 - hi) = SktM,
Where,     Ws = Weight of steam condensed, pounds per hour, Ww = Weight of water heated, pounds per hour, Q = Heat content of entering steam, B.t.u. per pound = h + xL where, h = heat of liquid and L = heat of vaporization, both corresponding with the pressure of the steam,  and x — dryness of steam entering the heater,
A0 = Heat content of condensed steam corresponding with its temperature Uj B.t.u. per pound, h2, hi = Heat content of water, B.t.u. per pound, corresponding with
its outlet and inlet temperatures tz and ti, S = Transmitting surface of heater, square feet, k = Transmission factor, and
tM — (t + U — 1>i — M -*• 2 = arithmetical mean temperature difference, where t = temperature corresponding with the pressure of the steam
The value of k is about the same as in surface condenser practice under like conditions.
In an open heater, the third member of the equations is disregarded and tQ — t%j A0 = ^2. The size of such a heater is based by the manufacturer on sufficiently low velocities of steam and water. Ratings will be found liberal.
Economizer.—The economizer is a feed-water heater using boiler flue gases as the heating agent. It usually consists of a single unit for an entire row of boilers, with a gas bypass to the stack. The surface is made up of vertical cast-iron tubes, 4 in. in diameter and 10 or 12 ft. long, through which the water rises. The outsides of the tubes are cleaned of soot by power-operated scrapers. Economizers obstruct the draft and often necessitate the use of fans.
The surface and effect are expressed by
0.24TFo(T"2 - T'd = Ww(h* - h,) « SKtM,
in which Wo — weight of gases, pounds per hour,
T'z, T'i = Inlet and outlet temperatures of gases, and*w = (T", + T\ - T2 - Ti)/2. The value of K is around 2.0; that of Wo is related to that of Ww- since with given fuel and given boilers there is per pound of coal a normal weight of feed water and a normal weight of flue gas. The value of Wo/Ws may be as low as 1.7 with the best operation and good coal; it may increase to three or four times this with bad operation. If C = pounds coal burned per hour, the proportion of (gross) saving due to the economizer is 0.24 Wo(T\ - T'i)/C. In common practice, T\ - T2>100. The value of TI should exceed 90° to prevent sweating of tubes.
Superheater.—This may be placed in the boiler setting, or separately fired. The Heine type (in the setting) has deflecting dampers to vary the gas flow. Other setting types should be placed in a zone where the gas temperature is at least 1,000°. Forcing the boiler decreases the amount of superheat. Provision should be made for flooding the superheater when it is not in use. It should be designed for at least 150°F. of superheat. Steam-pipe velocities may then be high. While increasing thermal efficiency of engines or turbines, superheat is injurious to metals and often interferes with lubrication. Plain, annular or corrugated (extension surface) tubes may be used. The design equation is